The dial looks nice. Generally speaking I find MK1 dial having more pronounced patina than MK2 dials.
The case seems the be quite polished (lugs appear to be very thin).
The insert seems to be slightly later one. I use to see them more with a flat top 4 type.

If you do a simple search on the classifieds you will find many 16660s and all those listings show the first 2 of the serial and many good pictures to compare the case to. 8.5 is to late in my opinion.

If you do a simple search on the classifieds you will find many 16660s and all those listings show the first 2 of the serial and many good pictures to compare the case to. 8.5 is to late in my opinion.

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Yes i know but i have also seen a 850M at the vrf dial archive with a MK1 dial.
This one :

The truth is no one knows for certain as Rolex keeps this information strictly confidential....bottom line is they manufacture/produce watches and over certain transitional periods, they have various parts all over their shelves and are known to reuse parts in excess inventory. Is that what happened here? I'm afraid no one can tell you for certain, all the vintage community has done (graciously thanks to everyone sharing their knowledge and experiences) is accumulate live examples/data points to guesstimate specifications in different time periods.

My opinion is that it seems too late for a mk1 (mine is earlier for sure, around 7.1 million) but I also believe no one can absolutely tell you with certainty that it isn't original (other than Rolex, who will not). If you like the watch, I suggest inspecting it to your satisfaction in terms of condition and not worry so much about this aspect....I do agree from that one pic the crown guards seemed to have hit the wheel a bit too hard.